The present invention relates to an apparatus for powering a sacrificial protective anode in a water storage tank and for reducing the rate of loss of the anode as a result of undesirable cathodic reactions and, more particularly, to an apparatus for providing operating power to a powered protective anode and to a circuit for reducing the protective anode current as a result of the cathodic effect of the metal-jacketed heating in an electric water heater.
A typical water heater includes a storage tank made of ferrous metal and lined internally with a glass-like porcelain enamel to protect the metal from corrosion. Nevertheless, the protective lining may have imperfections or, of necessity, not entirely cover the ferrous metal interior, such that an electrolytic corrosion cell may be established as a result of dissolved solids in the stored water leading to corrosion of the exposed ferrous metal and substantial reduced service life of the water heater. The water in the tank may be heated by gas or electric power and it is well known that uninhibited corrosion is substantially enhanced in the presence of hot water.
It is also well known in the art to utilize a sacrificial anode within the tank to protect against corrosion of the ferrous metal tank interior. The anode is maintained negative with respect to the tank, either by providing a passive anode constructed of a metal that is electrochemically more negative than the tank metal or by providing a source of electrical potential to establish a positive voltage differential between the anode and the tank.
In an electric water heater, an electric heating element is attached to the tank wall and extends into the tank to provide direct heating of the water. The heating element typically includes an internal high resistance heating element wire surrounded by a suitable insulating material and enclosed in a metal jacket such that the jacket is completely insulated from the internal heating element. Power for the heating element is typically supplied from a conventional 110 or 220 volt AC source. When the exterior metal jacket of the heating element is immersed in the water in the tank, it imposes an electrical load on the protective anode in the same manner as the exposed ferrous metal interior of the tank. As a result, the protective anode current is increased and the anode is subject to more rapid dissolution. Therefore, the life of the anode and thus the water heater are substantially shortened. In a typical electric water heater, less than half the protective anode current is needed to protect the tank interior with the remaining current resulting from the additional load imposed by the heating element jacket. However, the heating element jacket typically comprises or is plated with a metal more electropositive than the tank metal and thus does not require the same level of cathodic protection. In addition, heating elements are relatively inexpensive and easy to replace. In addition to the large current draw imposed on the protective anode by the heating element jacket, the heating element also creates a "shadowing" effect on any exposed interior portions of the tank in the vicinity of the heating element. As a result, anode current which might otherwise protect these areas of the tank flows instead to the heating element jacket and leaves the metal tank wall portions in this area with inadequate protection.
It would be most desirable, therefore, to reduce the electrical load which the heating element jacket imposes on the protective anode in an electric hot water heater. One way would be to simply electrically insulate the heating element jacket from the tank. However, the metal tank is typically grounded and, for safety reasons, a conductive path must be provided between the heating element jacket and the tank to provide a shunt for an overvoltage condition, such as would occur if damage to the heating element resulted in a short between the interior element wire and the metal jacket. Another solution to the problem would be to provide a resistance connection between the heating element jacket and the tank wall to reduce the anode current. However, to effectively reduce the anode current draw, the resistance would be too great to provide an adequate ground path in the event of an overload condition. It would also be possible to establish an impressed voltage differential between the heating element jacket and the tank wall, with the former maintained positive with respect to the latter. However, with the heating element jacket otherwise electrically insulated from the tank to allow maintenance of the potential difference, a conductive path for an overvoltage condition would not be available.
In a co-pending and commonly owned patent application entitled "Method and Apparatus for Reducing the Current Drain on the Sacrificial Anode in a Water Heater", there is described a system for applying a positive potential bias to the heating element jacket to reduce its current drain on the protective anode and simultaneously providing a low resistance current path to shunt a hazardous overvoltage current directly to ground. An electric hot water heater utilizing an electrically powered protective anode also requires a DC power source to provide the needed positive voltage bias to the anode.